fscache: Fix out of bound read in long cookie keys

fscache_set_key() can incur an out-of-bounds read, reported by KASAN:

 BUG: KASAN: slab-out-of-bounds in fscache_alloc_cookie+0x5b3/0x680 [fscache]
 Read of size 4 at addr ffff88084ff056d4 by task mount.nfs/32615

and also reported by syzbot at https://lkml.org/lkml/2018/7/8/236

  BUG: KASAN: slab-out-of-bounds in fscache_set_key fs/fscache/cookie.c:120 [inline]
  BUG: KASAN: slab-out-of-bounds in fscache_alloc_cookie+0x7a9/0x880 fs/fscache/cookie.c:171
  Read of size 4 at addr ffff8801d3cc8bb4 by task syz-executor907/4466

This happens for any index_key_len which is not divisible by 4 and is
larger than the size of the inline key, because the code allocates exactly
index_key_len for the key buffer, but the hashing loop is stepping through
it 4 bytes (u32) at a time in the buf[] array.

Fix this by calculating how many u32 buffers we'll need by using
DIV_ROUND_UP, and then using kcalloc() to allocate a precleared allocation
buffer to hold the index_key, then using that same count as the hashing
index limit.

Fixes: ec0328e4 ("fscache: Maintain a catalogue of allocated cookies")
Reported-by: syzbot+a95b989b2dde8e806af8@syzkaller.appspotmail.com
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/fscache/cookie.c

@@ -70,7 +70,7 @@ void fscache_free_cookie(struct fscache_cookie *cookie)
 }
 
 /*
- * Set the index key in a cookie.  The cookie struct has space for a 12-byte
+ * Set the index key in a cookie.  The cookie struct has space for a 16-byte
  * key plus length and hash, but if that's not big enough, it's instead a
  * pointer to a buffer containing 3 bytes of hash, 1 byte of length and then
  * the key data.
@@ -80,10 +80,13 @@ static int fscache_set_key(struct fscache_cookie *cookie,
 {
 	unsigned long long h;
 	u32 *buf;
+	int bufs;
 	int i;
 
+	bufs = DIV_ROUND_UP(index_key_len, sizeof(*buf));
+
 	if (index_key_len > sizeof(cookie->inline_key)) {
-		buf = kzalloc(index_key_len, GFP_KERNEL);
+		buf = kcalloc(bufs, sizeof(*buf), GFP_KERNEL);
 		if (!buf)
 			return -ENOMEM;
 		cookie->key = buf;
@@ -98,7 +101,8 @@ static int fscache_set_key(struct fscache_cookie *cookie,
 	 */
 	h = (unsigned long)cookie->parent;
 	h += index_key_len + cookie->type;
-	for (i = 0; i < (index_key_len + sizeof(u32) - 1) / sizeof(u32); i++)
+
+	for (i = 0; i < bufs; i++)
 		h += buf[i];
 
 	cookie->key_hash = h ^ (h >> 32);